questions about VFAQ's 'reading comp maps'

[99gst_racer]

OK, I just read VFAQ's "Reading Compressor Maps" article and I have a few questions.
LINK = http://www.vfaq.com/mods/Turbo-compressor-maps.html

In the acticle, it makes a reference to atmospheric pressure being 14.7 psi. Is that a standard number or does it vary? I mean, is it 14.7 everywhere or did they just use that figure as an example?

I also had a couple questions about the surge limit. It says "You also do not want to operate your turbo to the left of the surge line, the surge line the left most line. This only happens when you have a big PR at low airflow (big boost at low engine RPM). Surge is a condition similiar to stall over an airplane wing, the flow becomes turbulent over the compressor vanes and the compressor surges in and out of boost and all sorts of weird things happen. You'll definetly know if you're operating in the surge area."

What kinds of weird things happen? What would you look for or listen for? And also, give me some examples of when you would have a high Pressure Ratio and low airflow.

Thanks in advance. -Paul-

 

[99gst_racer]

OK, I found the answer to my first question. Atmospheric pressure at sea level is 14.7 psi, so that's where that figure came from.


OK, I got another question though.....
Here's what VFAQ says about figuring Pressure Ratios:
"For example, you're tooling along at 120mph with your foot to floor, and your boost gauge shows 14 psi (or .97 bar). The pressure ratio is:
[(your boost gauge reading)+(atmospheric)]/(atmospheric) = pressure ratio(PR)
[14psi+14.7psi]/14.7psi = 1.95 PR"

and I was reading another site that stated the following:

"Since atmospheric pressure at sea level is 14.7 psi, if you were to run 29.4 psi of boost, the pressure ratio would be 2. "

If you do the math, they cant both be correct. Which one if the correct one?

Also, if anyone has any answers to my questions on the surge limit in post #1, please feel free to explain.

 

[Staytuned]

Your first calculations are correct Paul. (Boost + 14.7)/14.7 = PR.

 

[psychlow]

14.7 PSI is at sea level, and decreases with altitude. This is because at high altitude, there is less atmosphere between you and space, so there's less air to press down on you.

(14.7 PSI + 14.7 PSI) / 14.7 PSI = 2.0.PR

The second calculation is also correct if you're talking about absolute pressure (PSIA). If your total air intake is 29.4 PSI, then yes, it is 2.0. If you're talking about gauge pressure: (29.4 + 14.7) / 14.7 = 3.0 PR.

Pressure ratio is really a bit more complicated than that, though.

PR = (Boost + Pressure Drop at Intercooler + Atmospheric Pressure) / (Atmospheric Pressure - Pressure Drop at Air Filter)

 

[CanadianTSi]

You don't really have to worry about surge on our cars as high boost and low rpm don't really go togther...

 

[psychlow]

You don't really have to worry about surge on our cars as high boost and low rpm don't really go togther...

Yeah, the only time you'll really need to worry about compressor surge is when your BOV isn't big enough (or operating well enough) to vent enough pressure off.

An easy way to find out if you're near the surge line is by logging airflow and doing the rough pressure ratio equation you posted. If you're running a 2.0 PR at 5 lb/min, you know something isn't quite right (unless you're running a 9B).

 

[99gst_racer]

Thanks for all of your help guys.....
I guess I have alot of math to do to plot where I'm at on my compressor map.